To angle
pickoff
amplifier
tension
signal



Aug. 3, 1965 w. 0. MULLINS, JR., ETAL 3,193,019

STABLE REFERENCE APPARATUS Filed March 1'7, 1961 ,2 Sheets-Sheet l INVENTOR. D. MULLINS JR. H. QUICK ATTORNEY WILLIAM WILLIAM i a on Nv mum Dom L L 1965 w. D. MULLINS, .IR.. ETAL 3,198,019

STABLE REFERENCE APPARATUS Filed March 17, 1961 2, Sheets-Sheet 2 CARRIER as gs? TO ANGLE PICKOFF 88 Q AMPLIFIER TENSION IN REF '0 SIGNAL 65 DEMODULATOR DEMODULATOR 32 POSITION FIG. 5 SIGNAL PICK ,6 32 oFF I 79 VARIABLE 3| El FREQUENCY 35 OSCILLATOR TENSION I I l SIGNAL 8| TENSION O I SIGNAL FIG.4 FIG. 3

TENSION F SIGNAL 6 OSCILLATOR INVENTORS WILLIAM D. MULLINS JR. WILLIAM H. QUICK ATTOR NEY United States Patent 3,l$ ti,tll9 STABLE REFERENQE APPARATUS William D. Mullins, .lr., Downey, and William H. Quick, La Mirada, (Ialiii, assi nors to North American Aviation, Inc.

Filed Mar. 17, 1%1, Ser. No. 96,611 16 Qlairns. (QR. 73-51%) This invention relates to stable reference apparatus, and more particularly concerns gyroscopic devices of the oscillatory type. The present invention is an improvement over that described in a co-pending application entitled Gyroscopic Apparatus of William D. Mullins, Jr. and William M. Scarborough, Serial No. 75,442 filed December 12, 1960 now Patent No. 3,106,847.

Because of imperfections in the vibrating string gyro and particularly those defects due to asymmetry and anisoelasticity of the string ends, together with external vibrations to which the string support may be subjected, the string path of vibration is actually an ellipse. This fact gives rise to a tendency for the string to follow its support with a slight lag. With the vibrating string operating nominally on its principal elastic axis, a small error causes an elliptical path to build up to a minor axis amplitude proportional to the product of error angle and non-uniformity of the string ends divided by string damping. With any but an ideal string, the basic string restoring force is nonlinear to some extent whereby the elliptical path causes the string plane to process toward a principal elastic axis at a rate proportional to the angle between the string plane and the principal axis. The elliptical motion of the string will cause the major axis of the ellipse to advance in the same direction as the motion of the string along the ellipse.

This rotation of the string plane is a precession of significant magnitude. l-owever, it has been found that there is a critical operating point wherein the direction of this precession reverses. By increasing frequency or decreasing amplitude of vibration it is possible to cause the string operation to move through the critical point to an operating point wherein the elliptical motion regresses, that is, to an operating point wherein the major axis of the ellipse processes in a direction opposite to the motion of the string along the ellipse. Thus, it will be seen that in operation at this critical operating point, the error precession (drift) connected with the elliptical path is a minimum. The critical operating point may be defined as the point between advancing and regressing characteristic of the elliptical motion where the elliptical orbit does not precess.

Applicants have discovered that by sensing when operation is at the critical point, frequency or amplitude of the drive may be so controlled as to maintain operation of the gyro substantially at this critical point. In particular, it has been found practical to measure the a-c component of tension, the tension modulation, which goes through a null and phase reversal at the critical point. By employing a signal representing this parameter which is indicative of the critical operating point, it is possible to regulate the string drive so that the string will operate virtually exactly on the critical point. With such a low sensitivity to elliptical motion, there is achieved a great increase in the instrument time constant. Accordingly, it is a primary object of this invention to maintain a vibrating string gyro at its critical operating point.

The problem of procession due to variation of tension and ellipticity of string motion is considered in the abovementioned co-pending application. It is recognized that variation of stored elastic energy such as exists due to tension variation of the string will cause the vibrating string to precess when vibration is in an elliptical path.

Such precession, of course, is an undesired error or drift. Thus, the co-pending application describes methods and structure for controlling the configuration of the apparatus itself in such a manner as to provide for operation with a minimum of variation of tension from the initial tension of the string. It has been found, however, that tension variation cannot be sufiiciently minimized by controlling the structure and configuration of the gyro apparatus. Thus, it is a further object of this invention to provide a practical means for minimizing variation of tension in a Vibrating string.

In carrying out the principles of this invention in accordance with a preferred embodiment thereof, there is provided a string mounted to a support for vibration and a driving means for effecting vibration of the string. String operation is maintained at the critical operating point by employing tension variation as a parameter indicative of the critical point. As the string is subjected to variation of tension when it is vibrating, there is provided a closed loop servo apparatus for servoing the tension variation toward zero so as to maintain operation at the critical point. According to one embodiment of the invention, there is provided a detector on one end of the string which is capable of sensing any varying axial pull exerted by the string. The detector is employed to control an automatic gain control circuit within a string drive oscillator whereby amplitude of the drive is changed in a sense such as to reduce the sensed tension variation.

Since precession due to ellipticity of the string path is a function of both frequency and amplitude of string oscillation, it is also Within the concept of this invention to utilize a signal representing tension variation to control the frequency of the string oscillation while maintaining a constant drive amplitude.

Also contemplated is the use of the string itself as a strain gage whereby variation in tension of the string is manifested as a variation in its electrical resistivity. Such a tension sensing arrangement permits a greater flexibility in string driving configuration.

From the foregoing it will be apparent that a major object of this invention is the provision of a closed loop servo for minimizing undesired precession connected with tension variation of the vibrating string. This and other objects together with many of the attendant advantages of the invention will be more readily understood in the light of the ensuing detailed description and drawings wherein FIG. 1 is a sectional View of the structure of an exemplary vibrating string arrangement, together with electrical circuitry therefor;

FIG. 2 illustrates an alternative drive arrangement for the apparatus of FIG. 1;

FIG. 3 illustrates an alternate control arrangement for the apparatus of FIG. 1;

FIG. 4 illustrates an alternate amplitude controller;

FIG. 5 shows an arrangement wherein the tension sensor comprises a strain gage bridge;

FIG. 6 depicts an exemplary compensation network; and

FIG. 7 is a sectional view of the magnet assembly of the structure of FIG. 1.

In the drawings like reference characters refer to like parts.

As illustrated in FIG. 1, a vibratile string 10 comprises a quartz fiber having enlarged integral end portions 11, 12, and a suitable metallic coating such as gold or chromium 13 to provide for the desired electrical conductivity of the string. The use of integral enlarged end portions to secure the string to the support avoids many of the difficulties of previous mounting arrangements wherein the string ends are cemented or soldered to the support. With the illustrated arrangement almost all elastic bending stress in confined to the area of cross-sectional transition rather than to the cement or solder which is notoriously less able to withstand such stress.

The string is mounted within a housing 14 which includes a driving disc 17 secured to the housing along an annular area of the disc by means of an annular support sleeve 18 integral with the disc and support member 19 which forms a fixed part of the housing 14. Enlarged portion 12 of the string is suitably secured as by cement to and within a closely fitting aperture located at the center of the drive disc 17. Preferably the disc is resonant at the drive frequency as described in connection with the driving diaphragms mentioned in the aforesaid cpending application.

A closed loop driving oscillator including disc 1'7 as a frequency control element thereof is provided substantially as described in the above-mentioned application. Disc 17 vibrates along the longitudinal axis of the string and drives the ends of the string along this axis. As is well known in the art, such end excitation not only causes the string to vibrate axially but also sets up vibrations in directions normal to the string axis. An electrode may be plated on the disc 17 if the latter is made of quartz, although it is preferredto make the instrument of a more easily worked material such as aluminum which itself is electrically conductive. Cooperating with the electrically conductive disc 1'7 is an annular pickotf electrode 3% mounted to the case by a suitable insulation 31. An electrostatic disc drive is provided by a second annular electrode 32 which cooperates with the electrically conductive disc 17. A signal from the capacitive pickoff electrode Bil is fed via a lead 33 and an amplifier 38 through an amplitude controller 34, for purposes to be described below, and thence through a power amplifier 35 to the electrostatic disc drive 32. to provide a closed loop oscillator substantial y as described in the aforesaid application. It deemed necessary or desirable, proper phase may be assured by the provision of a phase adjusting circuit 39 in the oscillator. The capacitative gap between electrode as and the disc 17 produces a change in capacity corresponding to the driven deflection of the end of the string. The capacitative gap is biased with a directcurrent voltage from a source 36 through a resistor 37 sufliciently large at the frequency of the drive that the capacitor operates at basically constant charge. Accordingly, the voltage on the plate 36 is proportional to the deflection of the string end 12.

At the other end of the case three thin resilient discs 40, 41 and 42 are mounted at the outer peripheries thereof to the case 14-. Disc 41 has an integral enlarged peripheral portion 43 which is fixedly secured to the case 14, while discs 4% and 4-2 have enlarged peripheral portions 44 and 45 suitably secured to the peripheral flange 43. A dielectric material as, 47 is provided between elements 43, 44- an 45 to provide electrical insulation between these elements. Each of the discs has an enlarged apertured inner hub portion 43, 49, d. The enlarged end 11 of the string is fixedly mounted (as by cement, solder, or the like) to and Within the apertures of hubs 48 and 49 to provide a spring mounting for this end of the string.

In order to compensate for certain asymmetrical characteristics of the string, it may be desired to initially twist the string as much as 90 degrees, for example. Thus, if deemed necessary or desirable, the periphery 43 of disc 41 together with the other discs afiixed thereto may be mounted so as to permit a limited manual rotational adjustment about the string axis. In such an arrangement slip rings are provided for the several electrical connections as described below in connection with the string frequency pickoif capacitor.

The discs are preferably formed of aluminum and made substantially thinner than drive disc 17. The lower disc 40 is in electrical contact with the conductive coating 13 of the string whereby an output lead 51 connected to peripheral portion 44 of disc 4% may be provided to supply the angle picltoit signal to the output amplifier (not shown) substantially as described in the aforesaid co-pending application.

As seen in FIG. 7, there is provided a magnet assembly 52 carrying string frequency pickoif capacitor plates 53, 54 adjacent the string substantially as described in the aforesaid co-pending application. The magnet assembly is a permanent magnet, as illustrated, and defines a reference direction with respect to which is measured the angular position of the plane of vibration of the string. The angle output signal derived from the string at lead 51 is produced by the component of string motion across the field of the magnet assembly. This signalris demodulated with reference to the string frequency pickoff capacitor signal by circuitry (not shown) more particularly described in the co-pending application. The capacitor plates 53, 54 are electrically connected (FIG. 1) to respective annular slip rings 55, 56 which cooperate with brushes 57, 58 connected to external leads (not shown). This arrangement permits rotation of the magnet assembly which is achieved by means of a manually rotatable pinion 59 meshing with gear teeth on the magnet assembly as and for the purposes described in the co-pending application. A set screw assembly dit is provided to lock the magnet assembly in position.

The end 11 of the string is resiliently mounted whereby its deflection will be proportional to the tension exerted by the string since the resilient discs 41 and follow Hooks law. Thus, a measure of the deflection will be a measure of the string tension. For the purpose of measuring string deflection and tension, there is provided a tension pickofr' capacitor formed by discs 41 and 42. The inner hub portion 5%) of disc 4-2 is spaced from the string end 11 whereby deflection of the string under tension will vary the capacitance provided between the closely spaced discs 41 and 42. It is noted that the electrically conductive string coating extends only slightly beyond hub 48 of disc it? in order to electrically insulate the angle pickofl circuitry from the tension pickoif.

While there is disclosed a string having an external electrically conductive coating, it will be readily appreciated that the string may take the form of a capillary having an internal electrical conductor to provide an arrangement which minimizes stress on the conductor.

such an arrangement the entire tension pickoff assembly including discs 4%, 41 and 42 would be turned over (upside down in the drawing) to position disc til at the string end for ease of connection with the string conductor.

It is possible that the flexing of the drive disc 17 about the annular supporting sleeve 18 thereof will initiate transmission of vibrations through the support body 14 to the peripheral flanges 43 and of the tension pickoif capacitor discs. Unless these discs deflect equally under such transmitted vibrations or accelerations, the latter will appear as a tension variation. Accordingly, the natural frequency of disc 42 is controlled so that this disc will deflect the same amount as disc 41 under the influence of acceleration whereby the sensitivity of the pickoif to vibrations transmitted through the case from the driving disc as well as sensitivity to external vibrations is reduced. Natural frequency of the disc 42 may be controlled by removing portions of the inner hub St} thereof.

Disc 41 provides a grounded side of the tension pickoif capacitor by means of a ground connection of its outer periphery 43. Disc 42 is connected through a resistor 61 to a source of direct current 62 which provides a bias for the pickoif in the same manner as that provided for the drive pickoff including electrode 30. At the output lead 63 of the tension pickolf there is provided a signal proportional to the variation in tension and varying at the frequency of the string drive. The tension variation signal is fed to an alternating-current amplifier 64 and thence to a conventional demodulator 65 which is phase referenced according to string drive positive by means of a signal obtained from the string drive picket? capacitor electrode 3t) and amplified in an amplifier as. If deemed necessary or desirable a phase adjuster may be employed between the amplifier and demodulator. in practice it is most convenient to employ a single amplifier in the place of amplifiers 38, as

to feed the two phase adjusters 39, 63. With this dcmodulating arrangement there is provided a signal of one polarity proportional to the amplitude of tension variation if the tension of the string is varying in phase with the drive motion. A signal of opposite polarity is provided at the output of demodulator as if the phase of the tension variation is displaced by 180 degrees from the phase of the drive. As explained above, this tension variation indicates the critical point of operation since it goes through a null and reverses at the critical point.

T he tension variation signal at the output of demodulator as is fed through a suitable servo compensation networx 67, as is well known to those skilled in the art, to provide a tension variation control signal on lead '75 which is fed to the amplitude controller 34 so as to control the gain of the driving oscillator and thus the amplitude of the drive signal fed to the driving electrode 32. It will be seen that a variation in tension from the fixed tension with which the string is passively mounted will result in a variation drive amplitude in a sense such as to decrease the sensed tension variation.

As illustrated in PEG. 6, the servo compensation network 67 may comprise a resistor 6'9 shunted by a series connected resistor 70 and capacitor ill.

An alternative string drive is illustrated in FIG. 2. In this arrangement the string is driven open loop at a frequency controlled not by a resonant driving disc but by the frequency of a carefully control ed oscillator 76 which drives the amplitude controller 34 to feed the drive signal through amplifier 35 to the electrostatic drive 32. In this embodiment the tension signal will be derived in a manner identical with that illustrated and described in connection with FIG. 1 and supplied to the amplitude controller 34 to vary the amplitude of string drive in a sense such as to minimize the detected variation in tension.

Amplitude controller 3 may comprise a conventional modulator having a carrier input from amplifier 38 and phase adjuster 39 in FIG. 1 or from the oscillator 'i'd in FIG. 2, with the tension signal such as that on lead 75 of HQ. 1 comprising the modulating input to the modulator. in such an arrangement the output of the amplitude controller is an A.-C. signal having an amplitude envelope controlled in accordance with the tension variation. It will be readily apparent that other and simpler types of amplitude controls are possible; for example, the tension variation signal may be employed to control bias of a variable gain tube or transistor. Alternatively, as illustrated in FIG. 4, a variable resistor may be employed as the amplitude control element. The arrangement illustrated in FIG. 4 is substantially similar to the drive arrangement described in the above-mentioned co-pending application and includes the resonant disc pickoff 31 pro viding a signal passed through a resistor 77 to an amplifier 78 having a thermistor '79 in its feedback loop. The output of the amplifier feeds a power amplifier Sb which drives the electrostatic drive 32. For the purpose of controlling the gain of the driving oscillator, the temperature of the thermistor '79 is varied with appropriate polarity of variation in accordance with the amplitude of the tension signal. This is achieved by means of a resistive heater element 81 physically located in close proximity to the thermistor '79 and electrically connected to receive the tension signal whereby the gain of amplifier 73 is controlled according to the tension variation. The speed of operation of this type of control has been found to be adequate for maintaining operation at the critical point.

As previously mentioned, the precession of the string caused by variation in tension is a function of both frequency and amplitude of the string drive. So, too, the

tension modulation itself is a function of both frequency and amplitude of string drive. In the previously described embodiment the frequency remains constant while amplitude of drive is servoed in order to zero the tension variation. As illustrated in FIG. 3, an ade of the drive will remain constant while the frequency is controlled in accordance with the tension variation signal. Conveniently, frequency control may employ an open loop drive such as illustrated in FIG. 2. Although closed loop drive is possible, it is more difficult to control frequency when employing a resonant type of drive. in the drive arrangement illustrated in PEG. 3 the electrostatic drive 32 is energized from amplifier under control of a variable frequency oscillator 35. The frequency of this oscillator is varied in an amount and in the proper sense to ef fect variation of he frequency of the string drive so as to reduce the detected tension variation.

The capacitor pickoli. illustrated in FIG. 1 is merely exemplary of several diiferent methods for measuring tension variation. It will be readily appreciated by those skilled in the art that the capacitor pickoif of FIG. 1 might be replaced by a magnetic or piezoelectric pickup. For example, the string end 11 might be directly affixed to the face of a piezoelectric element which is fixedly mounted to the string support whereby the element, as is well known, will provide an output signal proportional to string tension. Also contemplated Within the scope of this invention is the use of a strain gage bridge tension detector. As illustrated in PEG. 5, the upper end 11 of the string 14 is cemented to an upper portion of the support 14 while the lower end is mounted in a manner (not shown in FIG. 5) substantially similar to that illustrated in MG. 1. The electrically conductive metallic coating of the string, will exhibit, as is well known, a variation in resistivity with strain or stress of the string. In this arrangement a chromium string coating is preferred al though other coatings exhibiting suitable resistance-strain characteristics may be employed. Thus, the resistance of the string is indicative of its tension. Accordingly, the string may have one end connected to ground and other end connected to provide one leg of a bridge including resistors 86, S7 and 88 arranged in the illustrated bridge circuit and energized by suit-able source of A.-C. carrier 89. The bridge output comprising an A.-C. signal having an amplitude envelope representing string tension is provided by a transformer 99 having a secondary 9.1 which provides the input signal to a conventional demodulator 92. The demodulator 92 is referenced from the carrier 89 to provide at the demodulator output a signal which has an amplitude proportional to the input from secondary 9?. and a polarity depending upon the sense of such input. The strain gage bridge excitation could be direct current thereby eliminating demodulator 5 2. However, the A.-C. carrier excitation is preferred in order to avoid the possibility of the excitation signal interfering with the second harmonic of the main string pickoff signal derived from lead 5-1. Such interference is avoided by the use of a different carrier frequency in conjunction with the demodulator 92. The output of demodulator 92 is fed to the demodulator as and for the purpose described in connection with FIG. 1. This demodulator, as previously mentioned, is referenced by the string drive position signal and provides the previously mentioned tension signal whroh Will be fed as before to effect amplitude or frequency control of the string drive arrangement. While the strain gage sensing arrangement of FIG. 5 1s applicable with the driving arrangement illustrated in FIG. 1, itus more flexible than the capacitor detector illustrated in this figure since it will permit a symmetrical drive at both ends of the string in a manner described in the above-mentioned co-pending application. With a strain gage tension detector it is possible to mount bot-h ends String in vibratory resonant elements.

It will be seen that an improved stable reference of increased accuracy and greater freedom from drift errors is provided by the described method and apparatus for Zeroing tension variation of the vibrating string to maintain operation at the critical point. The tension variation is minimized by the use of a servo system including one of several diflerent types of tension sensitive devices to control the string drive. Adequate results are obtained from controlling either frequency or amplitude of string drive.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope or" this invention being limited only by the terms of the appended claims.

We claim:

:1. A stable reference apparatus comprising: a support, a vibratile string secured to said support at two points thereof, drive means for effecting vibration of said string nominally in a plane, said apparatus being subject to errors causing the string vibration to follow an elliptical path wherein a plane containing the string tends to precess in advancing or regressing direction around said path, said apparatus having a critical operating point between said advancing and regressing precession, means for sensing said critical point, and means responsive to said sensing means for controlling said drive means to maintain of said apparatus substantially at said critical point.

2. A stable reference apparatus comprising: a support, a vibratile string having enlarged integral end portions, means for securing said end portions to said support to provide a substantially fixed string tension, said string being subject to modulation of said tension when vibrating, and means for servoing said tension modulation toward zero.

3. A stable reference apparatus comprising: a support, a vibratile string, means mounting said string to said support with a substantially fixed tension, said string being subject to modulation of said tension when vibrating, and means for servoing said tension modulation toward zero, said last-mentioned means including a strain gage having said string as its sensitive element and string a drive means controlled by said gage.

4. A stable reference apparatus comprising: a support, a vibratile string, means mounting said string to said support with a substantially fixed tension, said string being subject to modulation of said tension when vibrating, and means for servoing said tension modulation toward zero, said last-mentioned means including a cap-acitative pickotf resiliently mounting one end of said string, and means under control of said pickofl for driving the other end of said string.

5;. A stable reference apparatus comprising: a support, a vibratile string, means mounting said string to said support with a substantially fixed tension, said string being subject to modulation of said tension when vibrating, and means for servo ing said tension modulation toward zero, sardlasnmentioned means including a variable gain string driving oscillator and a string tension p'ickofl connected to control the gain of said oscillator.

d. A gyro comprising a support, a tensioned vibratile string, one end of said string being secured to said support, means for resiliently mounting the other end of the string to the support for displacement longitudinally of the string, means for detecting deflection of said resiliently mounted string end, drive means for eflecting vibratron of the string, and means responsive to said detecting means for controlling the drive means so as to reduce the detected deflection.

7. A stable reference apparatus comprising: a support, a tensioned vibratile string, one end of said string being secured to said support, means for resiliently mounting the other end of said string to said support for displacement longitudinally of the said string, means for detecting deflection of said resiliently mounted string end, drive means for effecting vibration of said string, a demodulator having an input from said detecting means and phase referenced in accordance with string vibration, and means responsive to said demodulator for controlling said drive means so as to reduce the detected deflection.

8. A gyro comprising a support, a string stretched between and secured to two points of said support, means for sensing longitudinal deflection of one end of said string, variable gain means responsive to said sensing means for imparting vibration to said string end, said variable gain means including an amplifier and a heat sensitive resistor in circuit therewith, means for sensing tension variation of said string, a demodulator having an input from said tension variation sensing means and phase referenced from said deflection sensing means, a heater in proximity to said resistor, and means responsive to said demodulator for controlling said heater.

9. A gyro comprising a support, a string stretched between and secured to two points of said support, means for sensing longitudinal deflection of one end of said string, variable gain means including an oscillator for imparting vibration to said string end, means for sensing tension variation of said string, a demodulator having an input from said tension variation sensing means and phase referenced from said deflection sensing means, said variable gain means having a control input from said demodulator.

llti. A gyro comprising a support, a string stretched between and secured to two points of said support, means for sensing longitudinal deflection of one end of said string, variable frequency means for imparting vibration to said string end, means for sensing tension variation of said string, a demodulator having an input from said tension variation sensing means and phase referenced from said deflection sensing means, said variable frequency means having a control input from said demodulator.

ltll. A gyro comprising a support, a string stretched between and secured to two points of said support, means for sensing longitudinal deflection of one end of said string, variable gain means responsive to said sensing means for imparting vibration to said string end, means for sensing tension variation of said string, a demodulator having an input from said tension variation sensing means and phase referenced from said deflection sensing means, said variable gain means having a control input from said demodulator.

112. Gyroscopic apparatus comprising a support including at least one vibratory element, a string secured at two points thereof to said support and to said element and extending in the direction of vibration of said element, means fo sensing deflection of said element, driving means responsive to said sensing means for imparting a driving force to the element, means for sensing tension of the string, and means responsive to said tension sensing means for controlling said driving means.

13. A gyro comprising a support, a flexible drive disc fixed to said support along an annular area of the disc spaced inwardly from the periphery thereof and including a first electrode, an annular second electrode on the support adjacent to and cooperating with said first electrode to provide a pickoff capacitor, an annular third electrode on the support adjacent said first electrode, said third electrode providing an electrostatic disc drive in cooperation with said first electrode, an electrically conductive string having one end secured to said drive disc and the other end secured to a part of the support spaced from the disc, and feedback means interconnecting said annular second and third electrodes to provide a closed loop driving oscillator including said first-mentioned pickofl capacitor and said electrostatic disc drive.

14. A gyro comprising a support, an electrically conductive string having one end secured to a part of the support, a resilient disc secured at its periphery to said support at the other end of the string, said string being secured to said disc, said disc including an electrode, a pickoif plate fixed to the support adjacent to and cooperating with said disc electrode to provide a tension variation pickoif, said plate and said disc having the same natural frequency, drive means for vibrating the string, and means for controlling said drive means in response to said pickotf 15. A gyro comprising a support, a flexible drive disc fixed to said support along an annular area of the disc spaced inwardly from the periphery thereof and including a first electrode, an annular second electrode on the support adjacent to and cooperating with said first electrode to provide a pickotf capacitor, an annular third electrode on the support adjacent said first electrode, said third electrode being concentric With said disc and positioned outwardly of said annular area to provide an electrostatic disc drive in cooperation with said first electrode, an electrically conductive string having one end secured to said drive disc, first and second mutually spaced resilient discs secured at the respective peripheries thereof to said support, said string having an integral enlarged end portion at the other end thereof secured to said resilient discs, one of said resilient discs including an electrode, a pickotf plate fixed to the support adjacent to and cooperating with said one resilient disc to provide a tension variation pickofi, said plate and said one resilent disc having the same natural frequency, feedback means interconnecting said annular second and third electrodes to provide a closed loop driving oscillator including said first-mentioned pickoif capacitor and said electrostatic disc drive, a demodulator referenced from said first-menit) tioned pickoff capacitor and having an input from said tension variation pickofif, said feedback means including an amplitude controller responsively connected with said demodulator.

16. A stable reference apparatus comprising: a support; a vibratile string secured to said support at two points thereof; drive means for effecting vibration of said string nominally in a plane; said apparatus being subject to errors causing the string vibration to follow an elliptical path wherein the major axis of said elliptical path tends to precess in an advancing or regressing direction; means for sensing the direction of the major axis of said elliptical path; and means responsive to said sensing means for controlling said drive means to maintain the angular position of said major axis substantially at a predetermined direction.

References Cited by the Examiner UNITED STATES PATENTS 1,995,305 3/35 Hayes 73-517 X 2,466,018 4/49 Ferrill 73-505 2,546,158 3/51 Johnson 73-505 2,552,650 5/51 Rawlings 73-505 2,689,943 9/54 Rieber 73-517 X 2,974,530 3/61 Jaouen 73-505 RICHARD C. QUEISSER, Primary Examiner.

S. LEVINE, A. BLUM, JOSEPH P. STRIZAK, JAMES J. GILL, Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,198,019 August 3, 1965 William D. Mullins Jr. et a1 It is hereby certified that error appears in the above numbered patent requiring correction and corrected below.

Column 1, line 66, for "procession" read precession column 4, line 75 and column 5, line 1, for "positive" read position column 7, line 26, before "of" insert operation Signed and sealed this 17th day of May 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

2. A STABLE REFERENCE APPARATUS COMPRISING: A SUPPORT, AVIBRATILE STRING HAVING ENLARGED INTEGRAL END PORTIONS, MEANS FOR SECURING SAID END PORTIONS TO SAID SUPPORT TO PROVIDE A SUBSTANTIALLY FIXED STRING TENSION, SAID STRING BEING SUBJECT TO MODULATION OF SAID TENSION WHEN VIBRATING, AND MEANS FOR SERVOING SAID TENSION MODULATION TOWARD ZERO. 